Prokaryotes considered

As a college biology major during the 1970s I was taught that cells in which the genetic material is separated from the cytoplasm by a nuclear membrane – such as those of animals, fungi, plants, and protists – are called eukaryotes. In contrast, the DNA of bacteria is not bounded by such a structure, and hence these microbes are called prokaryotes, a name that means ‘before the nucleus’. This concept was accepted by biologists until the late-1970s, when Carl Woese used ribosomal RNA sequences to deduce the relationships among living organisms. He found that microorganisms previously thought to be bacteria, because they have no nucleus, were no more related to bacteria than to eukaryotes. He proposed that living organisms should be classified into three lineages, now called bacteria, archaea, and eukarya. Nevertheless, the prokaryotic classification is still used by many biologists. The following letter from Elio Schaecter, sent to TWiV, explains why: